Elevated pressure levels in the eye are caused by either elevated production of fluid or hampered outflow of fluid from the eye, or alternatively of combinations thereof. The intraocular pressure (IOP) in a mammalian eye is similar for a wide range of species and in the order of 11-21 mm Hg, such as 10-20 mm Hg, such as 12-18 mm Hg. For humans, a normal range is approximately between 11-21 mm Hg. It is considered elevated if exceeding 20 mm Hg, such as 21 mm Hg, such as 21-24 mm Hg, or 22-30 mm Hg for an extended time period. The level of the IOP is regulated by the formation of a fluid, aqueous humour (AH), originating from the blood and transferred via the ciliary processes into the posterior chamber of the eye. The AH passes the vitreous body and the lens in the posterior chamber and then through the pupil into the anterior chamber. From there most of the AH eventually flows to the irido-corneal angle and egresses the eye through the trabecular meshwork via the Schlemm's canal, the aqueous veins and the scleral and episcleral veins. There is an exchange of fluid and metabolites between the AH and e.g. the lens in the posterior chamber and the cornea in the anterior chamber. A minor portion of the AH enters the uveoscleral route, i.e. the iris, ciliary muscle and sclera and eventually mixes with locally produced tissue fluid before leaving the eye (Jerndal, Hansson & Bill, 1990; Oyster, 1999). The IOP is largely monitored by the outflow, while the formation of the AH in adult humans is considered to be less variable. The main regulators of the IOP are the endothelium of the trabecular meshwork, the juxtacanalicular endothelial meshwork and the inner wall endothelium of the canal of Schlemm (Liltjen-Drecoll 1998; Sacca et al 2005). The latter is of special importance in rectifying and monitoring the flow from the anterior chamber to the vascular system. In the endothelial cells, invaginations, named caveolae, are formed and filled with AH, and then chiefly transferred as giant vacuoles through the endothelial cells to eventually empty their content into the canal of Schlemm. Increased IOP results in the formation of a large number of such giant vacuoles and the opposite is true if the IOP is reduced (Jerndal, Hansson & Bill, 1990; Liltjen-Drecoll 1998). Additionally, these cells may tentatively have the ability to monitor their cell volume, thereby further influencing the paracellular leakage of AH (Starner et al., 2001). FIG. 1a shows a schematic figure of the human eye and FIG. 1b a scanning electron micrograph of the irido-corneal angle of a human adult eye.
The term intraocular hypertension is in medical practice used as a diagnosis for a chronic disease with an IOP exceeding 20 mm Hg, such as 21-24 mm Hg, such as 22-30 mm Hg. Patients with intraocular hypertension may suffer from that condition without developing any signs of sequel such as visual field loss or other signs of retinal and optic nerve abnormalities. Acute rise in the IOP may occur transiently at e.g. coughing, high workload, after a trauma to the eye bulb and a Valsalva's manoeuvre. There is normally a diurnal variation of the IOP, being highest in humans at early daytime. In an adult human, about 2-3 μL of AH is formed per minute, resulting in that the AH in an eye is renewed in about 1½ h. That means that the formation and outflow must be monitored within narrow limits to maintain the IOP within the normal range. The IOP must neither become too high or too low. The main functions of the AH are to provide nutrients, oxygen, ions and fluid to e.g. the lens and the cornea and to drain metabolites and debris. Further, the IOP and the AH interact to keep the optical properties and the shape of the eye. The production of AH may be disclosed by several methods, such as by determining the turnover of AH and its outflow. The intraocular pressure is accurately determined by e.g. tonometry. The presence of abnormal IOP, may be demonstrable in some glaucoma patients. Nonetheless, not all glaucoma patients do show signs of intraocular hypertension. The term intraocular hypertension must thus not be confused with the medical diagnosis of glaucoma. In contrast to intraocular hypertension, glaucoma is defined as a disease characterised by with time increasing detonation and eventually blindness due to progressive loss of retinal nerve cells and degeneration of the optic nerve. A wide range of conditions are included under the term glaucoma, all having in common that they eventually result in detonation of vision (Ritch et al., 1996). Thus, the diagnosis glaucoma relates to visual loss and not to the presence of abnormal IOP.
The IOP is mainly regulated by the outflow of the AH from the anterior chamber in the eye to the veins through the iridocorneal angle to the Schlemm's canal. If the egress of the AH from the eye is hampered or even blocked, the IOP will become elevated. The resistance to the outflow, normally resulting in an IOP in the range of 10-20 mm Hg, such as 12-18 mm Hg, is mainly localised to the trabecular meshwork and the endothelial lining of the canal of Schlemm. The endothelial cells in the trabecular meshwork and Schlemm's canal interact in the regulation of the outflow of AH (Alvarado et al., 2005; Jerndal et al., 1991). There is no direct communication between the anterior chamber on one hand and the Schlemm's canal, including the scleral and episcleral veins, on the other. In subjects with intraocular hypertension the trabecular meshwork is characterized by degeneration to a variable extent, accumulation of sheath-derived plaque material and increased resistance to outflow of the AH (Rohen et al, 1993). Further, accumulation of pigments and exfoliation material as well as of blood cells and clot may in certain cases as well add to and further hamper the outflow, elevating the IOP. Hypersecretion, i.e. excessive formation of AH, is a rare sole cause of intraocular hypertension. Thus, improved, sustained control of the turn over of AH, especially the control of the outflow of AH through the iridocorneal angle is of key importance in lowering and normalizing the IOP, in order to monitor intraocular hypertension.
Available therapies for the treatment of intraocular hypertension in clinical practice aim to preferably increase the outflow of AH, but some drugs, such as e.g. dorzolamide and brinzolamide, decrease the AH production. A few of the presently used drugs influence both the formation and the outflow paths. It ought to be stressed that the composition as well as the turn over of AH is of importance as AH supplies e.g. the lens and the cornea with nutrients, fluid and oxygen and takes care of formed vast products. Side effects are prevalent with presently used drugs and often hamper adequate treatment. Additionally, there are subjects with intraocular hypertension, who are not possible to treat adequately by available drugs. Thus, there is a need of therapeutic approaches for lowering the IOP to normal levels.
The antisecretory protein is a 41 kDa protein, originally described to provide protection against diarrhoeal diseases and intestinal inflammation (for a review, see Lange and Lönnroth, 2001). The antisecretory protein has been sequenced and its cDNA cloned. The antisecretory activity seems to be mainly exerted by a peptide located between 1-163, or more specifically, between the positions 35 and 50 on the antisecretory protein amino acid sequence. Immunochemical and immunohistochemical investigations have revealed that the antisecretory protein is present in and may also be synthesized by most tissues and organs in a body. Synthetic peptides, comprising the antidiarrheoic sequence, have been characterized (WO 97/08202; WO 05/030246). Antisecretory factors have previously been disclosed to normalize pathological fluid transport and/or inflammatory reactions, such as in the intestine and the choroid plexus in the central nervous system after challenge with the cholera toxin (WO 97/08202). Addition of antisecretory factors to food and feed was therefore suggested to be useful for the treatment of oedema, diarrhoea, dehydration and inflammation in WO 97/08202. WO 98/21978 discloses the use of products having enzymatic activity for the production of a food that induces the formation of antisecretory proteins. WO 00/038535 further discloses the food products enriched in antisecretory proteins as such.
Antisecretory protein and fragments thereof have also been shown to improve the repair of nervous tissue, and proliferation, apoptosis, differentiation, and/or migration of stem and progenitor cells and cells derived thereof in the treatment of conditions associated with loss and/or gain of cells (WO 05/030246).
The present inventors have now surprisingly found that antisecretory factor protein, homologues and peptide fragments derived thereof reduce the resistance to outflow of the AH through the iridocorneal angle of the eye to the venous system. This presents a new therapeutical application for antisecretory proteins, homologues and fragments thereof, i.e. the use of such proteins and fragments in the preparation of medicaments for the treatment and/or prevention of intraocular hypertension.